Process for cathodically sputtering magnetic thin films



Feb. 7, 1967 L. I. MAISSEL ETAL PROCESS FOR CATHODICALLY SPUTTERING MAGNETIC THIN FILMS 5 Sheets-Sheet 1 Filed Oct.

LEON Iv MAISSEL BARRY L FLUR BRUCE I BERTELSEN mkm ATTORNEY Feb. 7, 1967 L. MAISSEL ETAL 3,303,116

PROCESS FOR CATHODICALLY SPUTTERING MAGNETIC THIN FILMS Filed Oct. 9, 1964 5 Sheets-Sheet 2 PULSE PROGRAM WRIT E"I" READ"? WR|TE"0" READ"0" FIG. 3

I i I Feb. 7, 1967 L. l. MAISSEL ETAL Filed Oct. 9, 1964 5 Sheets-Sheet 3 TABLE I GLASS SUBSTRATE) SUBSTRATE Ho HKo B 0 THICKNESS (OERSTEDS) (OERSTEDS) (DEGREES) (DEGREES) 125 MIL 2.5 5.5 10 110 50 MIL L8 5.0 2- 6 1 4-i8 5 T02 MIL 1.8 5.0 1-2 f1-i5 G 8 TABLE I S UTTER RATE OF (A) NO BIAS (B) NEGATIVE BIAS (*(OOV) POTENTIAL DEPOSITION (VOLTS) (AD/SEC) Ho(0e) HKo(oe) B A H0(oe) HKo(oe) B A United States Patent 3 303 116 rnocnss FOR CATrIoDICALLY SPUTTERING MAGNETIC THIN FILMS Leon I. Maissel, Barry L. Flur, and Bruce ll. Bertelsen, Poughkeepsie, N.Y., assignors to International Business Machines Corporation, New York, N.Y., a corporation of New York Filed Oct. 9, 1964, Ser. No. 402,800 6 Claims. (Cl. 204-192) This invention relates to magnetic films, and, in particular, to magnetic thin films formed by cathodic sputtering, and, to the products resulting therefrom.

With the discovery by M. S. Blois, Jr., in 1955, that thin films of 80:20 (by weight) nickel-iron, when evaporated in the presence of a magnetic field, exhibit uniaxial anisotropy, both the academic community and industry have initiated large efforts to study these films. One reason for this is thatthese magnetic thin films potentially offer both engineering and economical advantages over present storage and switching devices, in data processing and computermachines. The anisotropy induced in these films provides ,an easy axis of magnetization aligned parallel to the externally applied field, along which two stable states corresponding to positive and negative remanence are available; and, a domain structure which allows rapid rotation of the magnetic remanence from one stable state to the other.

Storage or switching of intelligence is achieved by mag netizirig a particular element or bit, in an array of such elements, into either one or the other of its stable states. Rotation of the magnetization remanence takes place, upon application of the required switching fields, from one stable state to the other, in short periods of time, in the order of nanoseconds Characteristics such as these lend themselves to the applications as heretofore described.

Various techniques are available for preparing magnetic thin film devices that exhibit uniaxial anisotropy. These include: vacuum deposition, electroplating, chemical reduction, pyrolytic methods, and cathode sputtering. The first two of these methods have received wide attention in the literature, while the chemical reduction process or electroless plating involves the reduction of metal salts such as those of nickel, iron, and cobalt, with hypophosphite on an active or catalytic surface. The pyrolytic method, a process which has not attracted the interest such as that focused on the others, entails thermally decomposing an appropriate metal-organic compound, such as the mixtures of the nickel and iron carbonyls. Cathoode sputtering, the last of the procedures listed, has received attention by M. H. Francombe and A. I. Noreika in J. Appl. Phys., vol. 32, p. 995, 1961, and by A. J. Noreika and M. H. Francombe in J. Appl. Phys., vol. 33, p. 1119,1962.

Briefly, in cathode sputtering, two parallel plates are mounted in spaced relationship in a chamber at a pressure of about 10- to 10- torr and the plates coupled to a DC. voltage of several thousand volts. The cathode is an 80:20 alloy of nickel-iron (the target), and, the substrate positioned on the anode. Positive ions are produced, in the glow discharge, which results between the plates, and these are accelerated toward the cathode ejecting from its surface atoms or molecules.

Although success has been achieved with some of these heretofore mentioned techniques, the implementation of a thin film magnetic device utilizing cathode sputtering remains a substantial problem. The magnetic characteristics and economic advantages that theory affords, in comparison to other techniques, such as evaporation, has not as yet been fully realized.

One reason for this is that cathodically sputtered films condense on a substrate surface with greater amounts of impurities than evaporated films. Further complications are encountered in achieving reproducibility in a sputtered magnetic film which arises from the difficulty in controlling substrate temperature, and from the distortion of the applied magnetic field which is brought about bythe pres-.v ence of the ferromagnetic target. To obtaintheinherent advantages of cathode sputtering predicted by theory and experience, such as compositional uniformity and ease of deposition rate control, and to reduce the competitive gap between sputtering and the heretofore mentioned other processes for fabricating a magnetic thin film, various techniques have been tried but with little success. Accordingly, it has been an object of considerable research, therefore, to provide a cathodically sputtered magneticthin film for adaptation in a storage or switching de-. vice with the properties attendantto cathode sputtering.

It is the principal object of this invention to provide an improved proces for cathodically sputtering magnetic thin films.

It is a further object of this invention to provide an im proved magnetic thin film.

It is yet another object of this invention to provide an improved process for cathodically sputtering magnetic thin film exhibiting a high degree of magnetic uniformity.

It is yet another object of this invention to provide an improved cathodically sputtered magnetic thin film device with controlled dispersion and low skew.

It is yet another object of this invention to provide an economical and commercially feasible process for cathodically sputtering magnetic thin films for use in data processing and computer machines.

What has been discovered is that the aforementioned objects are realized, in accordance with the invention, by subjectingthin foils or sheets of ferromagnetic material to ionic bombardment and collecting the sputtered atoms or molecules on a substrate while the film is subjected to a suitable bias. The procedure reduces the eifect of field distortion and impurity contamination, thereby resulting in the production of a uniform magnetic thin film exhibiting controllable values of coercive force, anisotropy, dispersion and low skew. In addition, in accordance with the, invention, coercivity and dispersion are alterable, one from the other, in a fashion contrary to whatwas previously observed. It was previously found empirically, with vacuum evaporated nickel-iron films, that coercive force, anisotropy, and, dispersion were mathematically related by essentially a linear equation, and a reduction in the value of dispersion resulted in a compara-blede crease in the coercive force. With the present invention, lower values of coercive force are available for given values of dispersion than heretofore observed. The opportunity to vary these magnetic properties as presented by the instant invention is of particular interest in they preparation of high dispersion films, such as those described in U.S. patent application Serial No. 334,858 to Bertelsen et al., which is assigned to the assignee of the instant application.

While cathode sputtering methods are available in the prior art which employ substrate negative bias, and fabri cate cathodes, by particular processes, to reduce impurity content, these prior art techniques leave cathode sputter ing a commercially unfavorable process for producing magnetic thin films. The substrate biasing method, as used in the art, is applied in such a manner that were it to be employed in the formation of a magnetic thin film, especially Where it is desired to collect the sputtered material on a metallic substrate, the resulting deposit would contain metallic atoms of the metal substrate. As to the I latter technique, the cathode fabricated to control its impurity content, emphasis has been placed in the prior art on utilizing vacuum melted or electrolytically formed ferromagnetic cathodes. But,,these have had little effect on lessening the distortion of the magnetic field which is applied to induce the desired anisotropy; the field has nonuniform longitudinal and transverse components, resulting in the presence of nonuniform localized anisotropy fields, inhomogeneous material, and in nonuniform magnetic properties. r v

Now, in accordance with the present invention, one of the principal factors which has caused nonuniform magnetic characteristics in a thin' film, that is, the tendency to trap and retain large amounts of impurity during deposition, is overcome. This problem is usually more severe for sputtering than for evaporation because of the bombardment of fixtures and inner surfaces of the vacuum system by high energy ions of the sputtering gas, which produces reactive gases, not easily detected, and because the ionized species present during the sputtering are generally'more reactive than their neutral counterparts, are maintained within operable levels by selectively applying anegative bias (relative to theanode) to the substrate surface. The bias, when employed in the appropriate mannenefiects the "removal or exclusion of the impurities from the depositing film; this is accomplished without appreciable sputtering of the substrate material. s i

' The application of the' film bias in conjunction with a target of thin 'foil or sheet of ferromagnetic material, affords additional advantages. Since the target is thin, relative to the other elements of the cathode sputtering apparatus, the magnetic field distortion usually associated with the target is negligible compared to the externally applied field. The magnetic thin films that are cathodically sputtered in accordance with the present invention provide a sputtered magnetic thin film with uniformity of properties heretofore not available.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention as illustrated in the accompanying drawings;

In the drawings:

FIG. 1 is a schematic representation of the cathode sputtering apparatus utilized in the preparation of a magnetic thin film.

FIG. 2 is a schematic representation of a storage bit cell.

FIG. 3 is a typical pulse program utilized in the operation of the storage device of FIG. 2.

FIG. 4 is a schematic illustration'of the clip used to bias the film condensing on a nonmetallic substrate.

FIG. 5 is a schematic representation of the microscopic variance of the magnetization vector from the intended easy direction of magnetization to illustrate skew and dispersion. i

FIG. 6 is a schematic illustration of a storage bit cell such as depicted in FIG. 2 illustrating skew on the device.

FIG. 7 presents data illustrating the effect of target thickness on the magnetic properties of a sputtered film.

FIG. 8 is a table illustrating the effect of film bias on the magnetic properties of a magnetic thin film as compared to a film formed without said bias.

FIG. 9 is a 5 x 5 centimeter square film with the numerals thereon depicting the regions at which the magnetic properties of a magnetic thin film were measured to evaluate their uniformity as given in FIGS. 10 and 11.

FIG. 10 is a table illustrating the effect of film bias on the uniformity of a sputtered magnetic thin film as compared to a sputtered magnetic thin film formed without said bias.

FIG. 11 is a table illustrating the effect of a delayed bias on the magnetic properties of a thin film as compared to a cathodically sputtered magnetic thin film formed with a continuous film bias.

FIG. 12 is a table illustrating the effect of selectiv gaseous impurities added in the presence of a regulated bias on the magnetic properties of a cathodically sputtered magnetic thin film.

FIG. 13 is a graph of the magnetic parameters H H and B versus temperature.

Now, speaking generally as to the conditions heretofore described, regarding the cathode sputtering of a magneticthin film, reference is .made to FIG. 1, for convenience, which schematically illustrates the general type of apparatus, depicted as numeral 10, utilized inthe practice of the invention. Apparatus 10 includes a first electrode 2, the cathode assembly formed to also function as a heat sink, in that the lower portion 2a is added to the mass of the upper portion 2b. Rapid withdrawal of thermal energy from the face of the electrode is facilitated by the high conductivity of 2a and 2b andthe large radiating surface dissipating heat to the cooling shield 8, hereafter described. Bonded to the surface of cathode assembly 2 is a thin foil of ferromagnetic material 4, the target. Coupled to the cathode assembly 2' is the negative lead 6 of a voltage source (not shown). A shield 8, having cooling coils 7 about its periphery, is positioned around cathode assembly'2, within the Crookes dark space distance from the cathode. The Crookes dark space is a well known term in the art and is described in Vacuum Deposition of Thin Films by L. Holland, pp. -82 (1961 ed.)

"Below cathode assembly 2 is placed, in substantially parallel spaced relation thereto, anode assembly 14 which includes a housing through which cooling fluid 16, such as water, flows by way of inlet 18 and outlet 20. On the surface of the anode assembly is support 22, preferably glass, which serves to prevent substrate 24 from making contact with the anode. In the particular apparatus utilized, a spacing of 2.5 centimeters is maintained between cathode and substrate, but any convenient spacing is permissible, providing it is' maintained at a distance greater than the dark space distance. Anode assembly 14 is grounded via a lead 26 while the film that condenses on substrate 24 is biased as required via lead 28 coupled to the support 22.

Positioned between cathode assembly 2 and anode assembly 14 is rotatable shutter 30 which is placed between cathode 2 and anode 14 during the presputtering cleaning of the cathode. This is done to assure the removal of all contaminants from the surface of the ferromagnetic target. Once the precleaning step is performed, shaft 32 rotates shutter 30 away from its station between cathode 2 and anode assembly -14 to leave the ferromagnetic target 4 facing anode assembly 14.

Enclosing the'electrodes is hell jar 34 which, in the particular arrangement employed, has a diameter of about 18 inches. Bell jar 34-rests on base member 36 which contains two ports 38 and 40. The first port 38 is an inlet for a suitable gas via conduit 38a and control valve 38b. Argon, for example, furnishes the necessary ionized particles for bombarding the surface of the ferromagnetic foil. The second -port 40 serves to connect a second conduit 40a which, in turn, is controlled by a valve 40b, and, is coupled to a vacuum pump 42. It is usual to maintain the environment within the bell ja-r at a pressure in the range between 10- to 10* torr. Two coils 44a and 44b are mounted externally in hell jar 34 to provide a uniform magnetic field in the vicinity of the glow discharge, the coils being arranged to induce a magnetic field parallel at the surface of the substrate. To maintain a uniform field over the substrate surface requires relatively large coils.

By way of general example, a vacuum melted and rolled 12.5 centimeter square 81:19 nickel-iron sheet is bonded to the surface of the cathode assembly 2 and the vacuum system pumped down to less than 1 X 10 torr. Substrate 24 is mounted on support 22. Desirable materials for substrates are metals such as silver, copper, aluminum,

or the like, or a nonmetallic, such as glass. Where metal is used as the substrate, lead 28 for biasing the film need only make contact with the bottom surface of the metal substrate. But, where a nonmetalli-c such as glass is the substrate, a clip is used as depicted in FIG. 4, lead 28' is passed through apertures 23 provided in glass support 22' and coupled to the base 25 of clip 27' which encases the bottom and side surfaces of the glass substrate. The elbows 29 of clip 27' contact the periphery of the surface upon which the sputtered material condenses. In those instances where it is desired to maintain a continuous bias on the film during the sputtering process, a land 21', which is a thin line of metal of the same composition as the target, is predeposited on the surface of the substrate by any of the conventional techniques. Clip 27' is then coupled to the predeposited land to provide a conductive path over the substrate surface. As hereafter explained, the bias to the film is most effective after a continuous layer of sputtered material collects on the surface of the substrates. The continuous layer then serves as a conductive path to the clip 27, and dispenses with the need for predeposited land 21. The substrate is not clamped to support since this introduces stresses in the sputtered deposit, and stressing the film affects the uniformity of the magnetic properties. The bias clip, heretofore described, is mounted about the substrate to avoid the introduction of stresses in the device area of the film.

To provide the bombardment media for sputtering the target, argon is injected through port 38 to a pressure of approximately 0.1 torr, through conduit 38a and the regulation thereof maintained by valve 3812. With shutter 30 interposed between the cathode 2 and substrate 24, target 4 is cleaned by presputtering, as discussed above, to remove contaminants from the surface thereof. Following the cathode cleanup, shaft 32 rotates shutter 30 from the intermediate position between target 4 and substrate 24; Thereafter a potential of about 3000 volts, for example, is applied between the cathode and an anode at a current of about 175 milliarnperes. Once the glow discharge is initiated, a magnetic field of about 25 oe-rsteds is applied by way of coils 44a and 44b to induce the magnetic anisotropy in the desired direction in the sputtered material.

A magnetic thin film formed by the processes heretofore defined forms part of a storage matrix and one bit cell for such a matrix is shown in FIG. 2. Usually a series of these bit cells, generally depicted as numeral 50, are arranged in rows and columns with their associated conductors, that is, the word lines W and the commonbit sense lines BS disposed in such a manner that the conductors are substantially in quadrature one to the other. Bit cell includes a base portion 52, which may be glass, mica, metal or the like. Where metal is used, it serves also as the ground return for the lines W and BS thereby attaining closer inductive coupling to the device. Over base 52 layer 54 of chromium and layer 54' of silicon monoxide are deposited. The multiple layers of both chromium and silicon oxide are used to reduce the surface roughness and increase adhesion. The ferromagnetic film 56 is placed over layer 54; drive lines W and BS complete the device. Arrow 100 in the device represents the easy direction of magnetization which is parallel to the drive line W while arrow 200 represents the hard axis with the drive lines BS being parallel, or in other words, transverse to arrow. Bit cell 50 is word organized with the word lines W upon activation, furnishing a field transverse to the easy direction of magnetization of sufficient magnitude to rotate the magnetization 90 from the easy axis, while bit sense line BS upon activation, produces a field parallel to the easy axis 100.

For purposes of discussion, to illustrate the operation of the cell 50, assume that the remanent magnetization representing data is oriented along the direction of arrow 100. With a field along the drive line W the magnetization vector rotates away from the easy axis arrow toward the hard axis arrow 200. Upon activation of the drive line BS depending upon the polarity of the applied field (note that the bit line is activated before and deactivated after the word pulse) the magnetization vector falls either toward 100 or 100"; the state assumed upon cessation of the word pulse determines the polarity of the intelligence to be stored, that is, in binary nomenclature whether a binary 1 or a binary 0 is stored. Sensing of this stored information is achieved with activation of the drive line W during the rise time of the word pulse.

FIG. 3 depicts a typical .pulse program for energizing the drive lines W and BS discussion of which may lend to the understanding of the operation of bit cell 50. To store a binary l, first a pulse of positive polarity is applied along the drive line W driving the stored intelligence toward the arrow 200. Were information previously stored in the bit cell, a sense amplifier (not shown) coupled to the common-bit sense line BS would detect a signal such as that indicated under FIG. 30. Following the activation of the word line with a field of approxiamtely 3 H, (oersteds), drive line BS is activated, having a field strength of about 0.5 H (oersteds), the time sequence for the activation of the pulses of both the word and bit drive lines illustrated in FIGS. 3a and 3b. With a positive pulse along line R8,, the magnetization vector rotates toward 200", thereby storing a binary 1. Were a binary 0 desired, the drive line is activated, with a positive polarity as heretofore described, but, the polarity of the field induced along the bit drive line BS is opposite to that of the polarity induced for the storage of the 1 resulting in the magnetization vector resting along at 200. The requirements of the bit pulses are that they are large enough to assure complete rotation either to the right or left of 200 but small enough not to disturb intelligence stored along adjacent bits. The word pulse program requires that the applied fields are large enough to drive all bits toward 200 which represents the hard direction of magnetization. In principle there is no upper limit to its magnitude. 1

Other modes of operating a storage device are available, as described in the heretofore referred to patent application of Bertelsen et al. Operation of this device is based on device 50 having two additional quasistable magnetization positions in a direction orthogonal to arrow 100. In FIG. 2, as previously, arrow 100 represents the easy axis, but now positions 200' and 200 of arrow 200, the hard axis of magnetization, are utilized as additional quasistable states. The stability of these positions, which is initially unstable, is brought about by mutual locking of the magnetization sub-zones into which the film decomposes after the magnetic field pulse has ended.

To operate the device with four stable states, the word pulse is activated with the appropriate field strength to rotate the magnetization vector from the easy axis toward the hard axis. Depending upon the polarity of the applied word pulse, the magnetization vector assumes either position 200 or 200". To rotate the stored information from the hard axis, arrow 200, to a position along arrow 100, activation of both the word W and bit lines BS is required. Reading is performed in a similar manner, as heretofore described, for the conventional mode; the output signals are sensed upon the leading edge of the applied word pulse, with the polarity of the sensed signal depicting the intelligence stored.

With the operation of these magnetic thin film devices in mind, it is seen that the magnetic properties of a bit cell such as: coercive force H anisotropy field H dispersion ,8, and skew a are of particular significance in the evaluation of a magnetic thin film for its potential as a storage medium. These terms are well known in the art and are widely described in the literature, such as, for example, in the article by H. J. Kump, The Anisotropy Fields in Angular Dispersion of Permalloy Films, 1963, Proceedings of the International Conference on Non- 1 Linear Magnetics, Article 125. But, to facilitate the dis cussion at hand, the terminology is briefly reviewed:

H Coercive force is a measure of the easy direction field necessary to start a domain wall in motion, a threshold for wall motion switching.

H Anisotropy field may be thought of as the force required to rotate the magnetization from its preferred direction of magnetization to the hard direction and, H is the anisotropy field as viewed on a microscopic scale.

5-Dispersion is conveniently defined with reference to FIG. 5 which shows a section of a magnetic thin film, as comprising the aggregate of microscopic magnetic regions n. Associated with each of the microscopic magnetic regions n i a magnetization vector 11'. Under ideal conditions, each of the magnetization vectors it, related to a microscopic magnetic region 11, is parallel one to the other with the vector summation thereof yielding the intended easy direction of magnetization depicted as arrow 300. But, owing to various imperfections and fabrication difficulties, some of which are hereafter discussed, the intended easy direction of magnetization, arrow 300, is not achieved. The mathematical mean of each of the magnetization vectors n give rise to a mean easydirection of magnetization designated arrow 302, and the angle, a, between the intended easy direction, arrow 300, and the mean easy direction, arrow 302, is skew, which is more fully discussed below. Now, the angle in which we find 90% of the microscopic magnetization vectors n of the microscopic magnetic regions n is dispersion, and that angle ,8 is graphically illustrated in FIG. 5 as the angle between the mean easy axis, arrow 302, and the boundary line, arrow 304, which includes 90% of the deviations of the magnetization vector n' from the intended easy axis of magnetization arrow 300. Measurement of dispersion is similar to that discussed in the article by T. S. Crowther, entitled Techniques for Measuring the Angular Dispersion of the Easy Axis of Magnetic Film, Group Report #51-2, M.I.T. Lincoln Lab., Lexington, Massachusetts (1959).

a-Skew is defined with reference to FIG. 6 where a storage device is schematically shown such as that illustrated in FIG. 2 but without the drive lines. Again, as in FIG. 2, the ordinate 200 is the hard axis of magnetization and the abscissa 100 the intended easy axis. As a result of the average 'of the local dispersions of the easy direction, in the individual magnetic regions, the summation of these yields an externally discernible average easy direction for the entire film which is designated a, the angle be tween the actual easy axis 102 and the intended easy axis 100, one defines as the macroscopic deviation of easy direction of magnetization from the desired reference whereas 5 is the microscopic deviation as brought out in the discussion of FIG. 5. Various causes have been given for the variation from the intended easy axis: inhomogenities of the magnetic field used to impart the desired anisotropy, magnetost'rictive effects, stresses and strains developed during the deposition, substrate surface scratches, and temperature gradients. With the present invention, low values of skew, that is at, are obtained.

In order to provide a full understanding of the invention, the following specific examples illustrating the cathode sputtering of a magnetic thin film and the product resulting therefrom, in accordance with the invention, will now be presented. It is to be understood, however, these examples are merely illustrative and are not to be considered as limiting the scope of the invention.

Vacuum melted and rolled 12.5 centimeters square 81:19 nickel-iron sheet is bonded to the surface cathode assembly 2. Substrate 24, a glass plate of about 5 centimeter square, is positioned on support 22. From the work of B. J. Flur and J. Riseman reported in J. Appl. Phys, vol. 35, No. 2, p. 344 (1964), it is known that a thick ferromagnetic target distorts the unidirectional field in the vicinity of the substrate, and dispersion and skew are varied as a function of the amount of this distortion.

of the. sputtering apparatus are required.

With a 125 mil ferromagnetic target, the longitudinal field strength of the unidirectional field is reduced from 26 to between 11 to 8 oersteds over the substrate area, while the transverse component of the unidirectional field is reduced to 1.7 to 0.5 oersteds. With a 30 mil thick ferromagnetic target, the longitudinal component of the unidirectional field is maintained at about 15 oersteds and the transverse component markedly decreased. FIG. 7 presents a table in which the magnetic properties obtained there, 125 mil, a 30 mil, and a5 to 12 mil thick ferromagnetic targets are given to illustrate the effect of the ferromagnetic target thickness on field distortion.

In FIG. 7 the first column presents target thickness in mils; the second column presents H in oersteds; the third column H anisotropy field in oersteds; the fourth column [3 dispersion, in degrees; and, the last column a, skew, in degrees. It is to be noted that as the thickness of the ferromagnetic target is reduced, skew and dispersion are markedly affected, and is further to be noted, that for the fabrication of a magnetic thin film having the necessary uniform magnetic characteristics for use as a storage or switching device; ferromagnetic targets which are relatively thin in comparison to the other components In the particular embodiment shown, to realize a:2, ferromagnetic targets with thickness of 10 mils or less are preferable.

The effect of the bias on the substrate is now illustrated with further examples. 5 x 5 centimeter square silver-copper substrate, upon which is placed a vacuum deposited silicon monoxide layer having a thickness of about 2 microns, is used to collect the sputtered ferromagnetic material. It is common in the sputtering of magnetic thin films on glass substrates to predeposit a layer of SiO and most desirable when using metallic substrates. It is found that magnetic properties are markedly improved as a result of this underlayer of silicon monoxide; the silicon monoxide reduces surface roughness and may serve as a diffusion barrier between the substrate and the film. Where increased adhesion is sought, alternate layers of chromium and SiO are predeposited such as illustrated by layers 54 and 54' respectively of the device of FIG. 2. (See Silicon Monoxide Undercoating for Improvement of Magnetic Film Memory Characteristics, B. I. Bertelsen, J. Appl. Phys, vol. 33, No. 6, pp. 2026-2030, June, l962.) Of course, it will be recognized that while these particular examples were obtained with the use of a silver-copper substrate, other metal substrates are'usable, for example, aluminum, copper, molybdenum, tungsten, and the like. The highly conductive substrate is preferred in a magnetic thin film storage device since it provides a return path for the re quired drive line and permits better inductive coupling between the drive lines and the magnetic film. Now, in the data shown in FIGS. 8, 9, 10, 11, and 12, the cathode was precleaned for about 18 minutes at 3000 volts at a current of about milliamperes prior to the deposition of the magnetic film.

FIG. 8 presents data comparing the magnetic properties of thin film-s sputtered under conditions of film bias and no bias. The sputtering potential is given in volts, the deposition rate in Angstroms per second, H in oersteds, H in oersteds, ,8 in degrees, and 0c in degrees. The measurement of these parameters was taken from the center of the film sample. The films were sputtered to a thickness of about 800 Angstroms at different rates, as shown; and, with the initial substrate temperature maintained constant at approximately 275 C.; the temperature to which the film rises during deposition is a function of deposition rate. The sputtering rate was varied by changing the sputtering potential and current at a given pressure. Thickness was held constant by adjusting the sputtering time in accordance with the rates of sputtering at the different potentials. Note that all magnetic parameters increased with decrease in cathodic potential where no bias is used, but wherea negative bias is applied to the condensed film during its deposition, the contrary is observed. FIG. 8 shows that the magnetic properties of the films cathodically sputtered under suitable bias are essentially independent of the deposition rate, that the affect of the bias on the magnetic properties is very marked, and that the bias furnishes a better control over magnetic thin film properties than heretofore obtainable in cathode sputtering.

That the application of a selective film bias improves uniformity is brought out by the data of FIG. 10. H H B, and 0c are presented in the same units as heretofore discussed for the other figures. Each of the columns of FIG. 10 headed 1, 2, 3, 4, and 5, designates the edge points and center point respectively of a x 5 centimeter square film such as shown in FIG. 9: point 1 designates the bottom left corner of the film, point 2 the bottom right corner of the film, point 3 the upper right corner of the film, point 4 the top left corner of the film and point 5 the center position of the film. By measuring H H 5, and a, at each of these points, the effect of film bias during sputtering on uniformity is readily evaluated. FIG. indicates that the film bias generally improves a magnetic thin film and provides it with more desirable properties for adaptation as a storage or switching device.

Where magnetic thin films are cathodically deposited on metallic substrates, substantially covered with a nonmetallic such as silicon monoxide, selective application of the bias to the substrate is required than when the sputtered deposit is collected on a nonmetallic substrate like glass. What is encountered, if a film bias is applied continuously, during the sputtering process, is contamination of the sputtered film, which depreciates the storage characteristics of the magnetic thin film. The selective application of the film bias entails applying the bias potential to the metal substrate after the ferromagnetic target has been bombarded for a predetermined period, to permit the condensation of a continuous layer over the substrate. With nickel-iron-cobalt targets, for example, optimum results are attained with the film bias if it is applied after a thickness of about 75 Angstroms of sputtered film has collected. What is to be guarded in the cathode sputtering of a magnetic thin film is not the initial layer thickness to which the condensed film develops prior to activating a negative bias (with respect to the anode), but to see that there is a continuous layer over the substrate surface before initiating the substrate bias. The advantages of the delayed bias, the film bias activated after a continuous layer of sputtered material collects on the substrate, is readily seen from FIG. 11 where the magnetic properties are presented in the same units as heretofore used. The columns designated 1, 2, 3, 4, and 5 have the same significance, in relation to position upon the surface of the magnetic deposit, as brought out in the discussion on FIG. 10. FIG. 11 indicates that activating the film .bias after a continuous layer of magnetic film is deposited, advances further improvements in the cathode sputtering process which results in a more homogeneous film, containing fewer impurities and exhibiting greater uniformity of magnetic properties than with the application of a continuous bias.

In addition to the features and advantages heretofore described, the present invention offers insensitivity to impurity gases, such as oxygen, nitrogen, and the like, which normally prohibit the formation of useful films. This is observed from FIG. 12 where cathode sputtering was performed at a substrate temperature of 325 C., with a directly cooled cathode, in an argon atmosphere containing 0%, 0.25%, 0.4% and 1% by volume of nitrogen, or 0%, 0.25% and 1% by volume of oxygen. In these particular examples, the potential between cathode and anode was maintained at about 2000 volts with a current of 110 milliamperes and the bombarded deposit collected in SiO coated copper substrates. The

designated concentrations of oxygen and nitrogen were injected into the apparatus under conditions of both film bias and no bias, the bias potential being volts in all cases except one, where 200 volts was applied to the substrate in the presence of 0.40% N as indicated in FIG. ll. The data show that large impurity concentrations require a greater bias potential on the substrate (relative to the anode) in order to eliminate adverse afiects and bring about the desired magnetic prop erties, and a delayed bias provides additional advantages over the continuous bias, when the substrate is metal, as evidenced by the last example for 1% N as shown in FIG. 11. This further indicates that sensitivity to impurity contamination is decreased and the control over the resultant magnetic properties is enhanced with a film bias during cathode sputtering.

To realize the optimum properties available in accordance with the invention, the initial substrate temperatures are maintained within preferred limits. That this is desirable is demonstrated by FIG. 13 where H 5, and H are plotted against initial substrate temperatures. The ordinate of the graph is the magnetic parameter while the abscissa is the substrate temperature. As illustrated by the graph, H increases with temperature, H decreases slightly with increasing temperature, and [i is relatively insensitive to temperature up to about 400 C., then increases rapidly.

What has been described is a technique for producing magnetic thin films for use as storage and switching devices in computer applications, the films being the product of a cathodic sputtering process. Uniformity of magnetic characteristics is provided over the surface of the storage medium and means are provided for varying the various magnetic properties independently one from the other. As will also be apparent to those skilled in the art, the illustrated arrangement for the apparatus can be readily modified without departure from the principles of the herein described invention. For example, rather than cooling the anode, heat may be applied or a cooled cathode assembly used in place of the heat sink type. Although, in the discussion that has been presented, the ferromagnetic target materials heretofore referred to were of limited class, it will be recognized by those versed in the art that a variety of ferromagnetic materials are amenable to the process heretofore described, these include the ferromagnetic materials containing from 15 to 35% iron with the balance nickel; the nickel-iron-molybdenum alloys such as the 4 molybdenum-79 nickel-l7 iron, and the iron-nickel-cobalt alloys such as the 79 iron-17V: nickel- 2 A2 cobalt. Basically any magnetic alloy is available to the process. Likewise, practice of the invention is not confined to argon, other bombarding media like helium, neon, krypton, mercury and xenon are usable.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. A process for cathodically sputtering a magnetic thin film comprising the steps of:

positioning a target of a ferromagnetic alloy on the face of an electrode;

impressing a potential between said electrode and another electrode in approximately spaced relationship to said first electrode such that said first electrode is at a negative potential with respect to said other electrode;

applying an electrical bias to the sputtered material which collects on a substrate, said bias maintaining said film at a negative potential With respect to said other electrode but at a positive potential with respect to said first electrode; and

simultaneously inducing a magnetic field substantially parallel at the surface of said substrate and through said sputtered material.

2. A process, for cathodically sputtering a magnetic thin film of the type adapted for the storage or switching of intelligence in data processing or computer machines, comprising the steps of:

providing two electrodes in approximately parallel spaced relationship one to the other where said elec trodes are in an enclosure;

positioning on the face of one electrode a target of a ferromagnetic alloy;

placing about the face of said second electrode the anode, a substrate, which substrate is in spaced relation to said anode;

reducing the pressure about said first and second electrodes;

injecting a source of gaseous material between said target and said anode; applying a potential between said target and said anode such that said target is at a negative potential with respect to said anode, while applying a negative electrical bias to the sputtered layer collecting on said substrate with respect to said anode; and

simultaneously inducing a magnetic field substantially parallel at the surface of said substrate and through said sputtered material.

3. A process for cathodically sputtering a magnetic thin film of the type adapted for the storage or switching of intelligence, comprising the steps of: 7

providing two electrodes in approximately parallel spaced relationship one to the other in an enclosure; mounting on the face of one electrode a thin target of a ferromagnetic alloy; placing in proximity to the face of said second electrode, the anode, a substrate, where said substrate is spaced from the face of said anode by way of a support;

reducing the pressure about said first and second electrodes to a predetermined level;

injecting a source of gaseous material between said first and said second electrodes;

applying a potential between said target and said anode while applying an electrical bias to the sputtered film collecting on the substrate to maintain said sputtered film at a negative potential with respect to the anode but at a positive potential with respect to said target while simultaneously inducing a magnetic field substantially parallel at the surface of said substrate and through said sputtered material, the application of said potentials between said target and said anode causing the gaseous material to bombard the thin ferromagnetic target and sputter material from the target which collects on the substrate, thereby forming a magnetic thin film characterized by uniform magnetic properties. i

4. A process for cathodically sputtering a magnetic thin film for adaptation as'a storage or switching device in a data processing or computer machine, comprising the steps of:

providing two electrodes, in an enclosure, in approximately parallel spaced relationship one to the other; mounting on the face of one electrode a thin target of a ferromagnetic alloy; placing in proximity to the face of said second electrode, the anode, a substrate, the substrate being mounted on a support, which support is in turn mounted on the face of said anode;

reducing the pressure about said target and said anode;

injecting a source of gaseous material between said target and anode;

applying a potential between said target and anode such that said target is at a negative potential with respect to said anode while applying an electrical bias to said substrate such that said substrate is at a negative potential with respect to said anode but at a positive potential with respect to said target while simultaneously inducing a magnetic field substantially parallel at the surface of said substrate and through said sputtered material, the application of said potential across the target and anode causing gaseous material to bombard the ferromagnetic target and sputter ferromagnetic material from said target which collects on said biased substrate to form a magnetic thin film characterized by uniform coercive force, anisotropy field, dispersion and skew.

5. A process for cathodically sputtering a magnetic thin film adapted for the storage or switching of intelligence in data processing machines, comprising the steps of:

providing two electrodes in approximately spaced relationship in an enclosure;

mounting on the face of one electrode a thin target of a ferromagnetic alloy;

placing on a support Which, in turn, is mounted on the face of said second electrode the anode, a substrate, which substrate includes a metallic base and a superimposed coating of nonmetallic material thereover;

reducing the pressure about said target and anode;

injecting a source of gaseous material between said cathode and anode;

applying a potential between said target and anode such that the target is at a negative potential with respect to said anode, and, electrically biasing the substrate such that the substrate is at a negative potential with respect to the anode but at a positive potential with respect to the target while simultaneously inducing a magnetic field substantially parallel at the surface of said substrate and through said sputtered material, the application of said potential across the target and anode resulting in a gaseous mate rial bombarding the ferromagnetic target and sputtering ferromagnetic material from said target which collects on said insulated coated surface of said metallic base to form a magnetic thin film device characterized by uniform coercive force, anisotropy field, dispersion and skew.

6. A process for cathodically sputtering a magnetic thin film adapted for the storage or switching of intelligence in data processing and computer machines comprising the steps of providing two electrodes in approximately parallel spaced relationship one to the other in an enclosure;

mounting on the face of one electrode a thin target of a ferromagnetic alloy;

placing on a support a nonmetallic coated metal substrate which, in turn, is mounted on the face of said second electrode, the anode;

reducing the pressure about said target and anode;

injecting a source of gaseous material between said target and anode;

applyingta potential between said target and said anode such that the target is at a negative potential with respect to said anode whereby the gaseous material is caused to bombard the ferromagnetic target and sputter material from the same, which material collects on said coated face of said substrate; and

after a continuous layer of sputtered material collects on said coated face of said substrate, applying an electrical bias to said substrate such that said substrate is at a negative potential with respect to said anode but at a positive potential With respect to said target while simultaneously inducing a magnetic field substantially parallel at the surface of said substrate and through said sputtered material and continuing with the application of said potential between said target and anode to form a magnetic thin film on the coated face of said metal substrate to form a magnetic thin film characterized by uniform coercive force, dispersion, anisotropy field, and skew.

References Cited by the Examiner UNITED STATES PATENTS 3,021,271 2/1962 Wehner 204l92 3,077,444 2/ 1963 Hoh 204l92 3,117,065 1/ 1964 Wootten 20420 OTHER REFERENCES Frerichs, Journal of Applied Physics, May 1962, pp. 1898-1899.

JOHN H. MACK, Primary Examiner.

R. MIHALEK, Assistant Examiner. 

1. A PROCESS FOR CATHODICALLY SPUTTERING A MAGNETIC THIN FILM COMPRISING THE STEPS OF: POSITIONING A TARGET OF A FERROMAGNETIC ALLOY ON THE FACE OF AN ELECTRODE; IMPRESSING A POTENTIAL BETWEEN SAID ELECTRODE AND ANOTHER ELECTRODE IN APPROXIMATELY SPACED RELATIONSHIP TO SAID FIRST ELECTRODE SUCH THAT SIAD FIRST ELECTRODE IS AT A NEGATIVE POTENTIAL WITH RESPECT TO SAID OTHER ELECTRODE; APPLYING AN ELECTRICAL BIAS TO THE SPUTTERED MATERIAL WHICH COLLECTS ON A SUBSTRATE, SAID BIAS MAINTAINING SAID FILM AT A NEGATIVE POTENTIAL WITH RESPECT TO SAID OTHER ELECTRODE BUT AT A POSITIVE POTENTIAL WITH RESPECT TO SAID FIRST ELECTRODE; AND SIMULTANEOUSLY INDUCING A MAGNETIC FIELD SUBSTANTIALLY PARALLEL AT THE SURFACE OF SAID SUBSTRATE AND THROUGH SAID SPUTTERED MATERIAL. 